To make a purchase, simply write a check for the purchase amount to a merchant and sign the check. Your credit card company pays the merchant when the check is cashed, and the amount of the check is deducted from your card's credit line.

Write a check to yourself and cash it -- you're now using a convenience check as a cash advance.

Using a convenience check, or two or three, for purchases or cash advances can wipe out your credit line pretty fast.

"Convenience checks are just that. They're convenient," says Gail Cunningham, vice president of public relations for the National Foundation for Credit Counseling.

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"But what consumers need to realize is there's always a cost of credit."

And the cost of credit for a convenience check can be pretty steep.

The interest rate charged on convenience checks tends to be higher than the interest rate charged for credit card purchases, and you also will be charged a fee, typically 3 percent or 4 percent of the amount that you borrow, says Linda Sherry, director of national priorities at Consumer Action.

"When you write a convenience check, you will typically be charged the cash advance interest rate, which may be higher than your regular rate, and a cash advance fee," Sherry says. "Usually interest begins to accrue immediately. And there may be a charge to stop payment on a convenience check."

The interest rate you pay on a convenience check could be twice as high as the rate you would pay for a credit card purchase, Sherry says.

To encourage consumers to use convenience checks, credit card issuers might offer a lower promotional rate for a limited time or offer to waive the fee on convenience checks.

"Sometimes, you'll get special offers that waive the fee and offer a special interest rate," Sherry says.

Should you ever use a convenience check?

A convenience check with a low promotional interest rate could be used to pay off a balance on another credit card with a much higher interest rate.

"That's not a bad move," says Ulzheimer, especially if the consumer plans to pay off the entire debt during the lower-rate promotional period.

But before using a convenience check, Cunningham encourages consumers to have a repayment strategy in mind.

"Have a repayment plan in mind before you sign on the dotted line," Cunningham says. "Make a contract with yourself as to when you will pay off the money."

Study the terms and conditions of a convenience check offer carefully, and read the fine print, Ulzheimer says. "The terms may not be as attractive as you think."

Here are some important questions to ask before using a convenience check.

What's the APR? What annual percentage rate, or APR, will you pay on a purchase, cash advance or balance transfer made with a convenience check?

Is there a promotional period with a lower interest rate for a convenience check? How high will that APR climb once the promotional period ends? Will you be able to pay off the amount you spend with a convenience check before the higher interest rate kicks in?

"The key issue is to know when in the billing cycle the introductory rate expires," says Robert Manning, author of "Credit Card Nation" and president of DebtorWise Foundation. "So you'll know when you'll have to pay it off."

Examine each individual convenience check carefully. APRs on convenience checks can vary from check to check. Don't assume a batch of convenience checks all have the same rate, Manning says.

Sherry recommends calling the issuer before using a convenience check to verify the APR.

How much is the fee? Take a close look at the fee you'll be charged for each convenience check that you use. Is it 3 percent or 4 percent of the amount you borrow with the check? Will the issuer waive the fee if you use the checks during a promotional period? Calculate the cost of fees carefully.

"Cash advance fees are very typical on these checks, up to 3 (percent) or 4 percent of the total you write," Sherry says.

Is there a grace period? With some convenience checks, issuers will begin charging interest on the checks as soon as you use them.

"In some situations, you begin paying interest immediately rather than after a grace period," Ulzheimer says.

How will this affect my credit line? Be sure you understand the impact using convenience checks will have on your card's credit line. How much of your credit limit does an issuer allow for convenience checks? You may have a lower limit than you realize for convenience check transactions.

"You do not have the same credit line for cash-related advances as you do for charges," Ulzheimer says. "Call and double-check."

If you decide to pass on an offer, get those unused checks to a shredder as soon as possible.

"They should go immediately into your shredder," Ulzheimer says. "Tossing them in the garbage is an invitation to (account takeover) identity theft. It's like tossing your credit card in the trash without shredding it."

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